The long range goal of this work is to understand the function, development, and differentiation of Ly-1 B cells. These B cells constitute a small population first identified by their expression of the pan T cell marker CD5. They differ in many respects from the majority of B cells that do not express CD5 (conventional B cells), disproportionate representation among B cell neoplasias. We have previously identified a bias in immunoglobulin variable (V) gene use in peritoneal Ly-1 B cells due to antigen selection. 5-15% of Ly-1 B cells are specific for the hapten phosphatidyl choline (PtC) and use either of two heavy (H) and light (L) chain V gene combinations. No conventional B cells with this specificity are detected. We propose to extend this work by examining the use of other V genes for evidence of positive and negative selection. This will be done by comparing variable H gene expression by Ly-1 and conventional B cells using cDNA libraries derived from cells of each population, and by examining V gene use in Ly-1 B cell hybridomas. We also propose to examine the basis for the absence of detectable numbers of PtC specific conventional B cells by creating a transgenic mouse model in which the rearranged H and L chain genes for an anti-PtC antibody are introduced into the germline. The fate of conventional PtC specific B cells will be determine by flow cytometry. In addition, we will assess the expressed V gene repertoire of Ly-1 B cells from mice homozygous for the viable motheaten (me) mutation. These mice suffer from a severe autoimmune and immune deficiency disease derive from Ly-1 B cells, since all IgM producing B cells in (me) mice are Ly-1. We propose to determine whether the production of pathological autoantibodies is the result of selection by antigen as is the case in mice of other autoimmune strains, or polyclonal stimulation.